This invention relates, in general, to electronic speech enhancement systems and, more specifically, to dynamic gain control of voice signals.
In a variety of applications, it is desirable to receive and understand voice or speech communication signals in the presence of audio interference. Such speech signals may be derived directly from radio receivers, recordings, intercoms, or other sources of audio signals. The interference associated with the speech depends to some extent upon the nature of the speed signal and the environment from which it originated. Experience has shown that it is desirable to eliminate at least three types of noise interference signals when the speech-to-noise ratio is relatively low. It is desirable to eliminate tonal noises, which correspond to continuous and repetitive tone noises, such as engine whine and 60 Hz AC power hum. It is also desirable to eliminate impulse noises in the speech enhancement system which could originate, in this example, due to communication jamming signals or to local electromagnetic signal interference at the receiving site. A third type of noise, wideband noise, is often present when the signal is extremely weak and eliminating such noise by the speech enhancement system is highly desirable.
Modern state of the art speech enhancement systems usually operate in a digital mode wherein the analog speech signals are first converted into digital values by a sampling technique before being processed. Due to the inherent features of a digital system, it is desirable to maintain the signals applied thereto within a specified range of digital values. Applying a digital value too large may saturate the digital system, thereby adding distortion to the speech. Applying a digital value which is too small to the digital system lowers the resolution capabilities and quantization noise detracts from the performance of the speech processor. To alleviate this situation, it has been standard practice according to the prior art to apply the incoming, unenhanced speech signal to an automatic gain control (AGC) circuit which provides a relatively constant signal level for use by the speech enhancement system. However, since in many situations the noise energy present in a speech plus noise signal is many times greater than the speech contained within the signal, and since an AGC circuit responds to the total or composite signal, the amount of speech signal present in the constant output varies and is a function of the variation in the noise component of the input signal. For this reason, the voice signal remaining after the speech enhancement system removes the noise components from the signal processed by an AGC circuit, varies in amplitude and is not as desirable as a speech signal having a nearly constant level arranged over time where short time fluctuations correspond to the original speech amplitude fluctuations before being processed.
Therefore, it is desirable, and it is an object of this invention, to provide a speech enhancement system whereby the speech or voice signals provided at the output of the system have an amplitude more representative of the input speech amplitude than conventional prior art systems while keeping the speech signal averaged over time at nearly a constant level.